Organic electroluminescence (referred to as EL, hereinafter) devices are used for a planar light emitting member such as a flat panel display of wall televisions and a back light of displays and the development of EL devices has been widely conducted.
Light emission from an organic substance under an electric field was observed in 1963 by Pope as light emission from a single crystal of anthracene (J. Chem. Phys., 38 (1963) 2042). In 1965, Helfinch and Schneider succeeded in observing relatively strong electroluminescence of the injection type using a solution electrode system having a good efficiency of injection (Phys. Rev. Lett., 14 (1965) 229). Since then, studies on forming organic light emitting substances from conjugated organic host substances and conjugated organic activating agents having condensed benzene rings have been reported. As the examples of the organic host substance, naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyls, trans-stilbene and 1,4-diphenylbutadiene were shown. As the examples of the activating agent, anthracene, tetracene and pentacene were shown. However, these organic light emitting substances existed as a single layer having a thickness exceeding 1 μm and a high electric field was required for the light emission. Therefore, studies on a thin layer device using the vacuum vapor deposition process have been conducted (for example, Thin Solid Films, 94 (1982) 171). However, a device exhibiting a sufficiently high luminance for practical application could not be obtained although the use of the thin layer was effective for decreasing the driving voltage.
Tang et al. prepared an EL device having two very thin films (a hole transporting layer and a light emitting layer) which were laminated in accordance with the vacuum vapor deposition process and disposed between the anode and the cathode and succeeded in obtaining a high luminance under a low driving voltage (Appl. Phys. Lett., 51 (1987) 913 and U.S. Pat. No. 4,356,429). Thereafter, the development of organic compounds used for the hole transporting layer and the light emitting layer was conducted for more than a dozen years and the life and the efficiency of light emission sufficient for practical application could be achieved. As the result, the practical application of the organic EL device started in the area of displays of automobile stereos and portable telephones.
However, the luminance of light emission and the durability against degradation after the use for a long time are not sufficient for practical applications and further improvements are required. In particular, when an organic El device is applied to full color displays, it is required that the luminance be as high as 300 cd/m2 or greater and a half-life be as long as several thousand hours or longer with respect to each of R, G and B colors. It is particularly difficult that these properties are achieved with respect to blue light. For the emission of blue light, the gap of the light emitting layer must be as great as 2.8 eV or greater. The energy barrier in the hole injection between the hole transporting layer and the light emitting layer is great and the intensity of the electric field applied to the interface is great. Therefore, stable hole injection cannot be achieved by using a conventional hole transporting layer and the improvement has been desired.
When application of an organic EL device to automobiles is considered, conventional organic EL devices have a problem in storage at a high temperature such as a temperature of 100° C. or higher. Conventional hole transporting layers have low glass transition temperatures and it was found that overcoming this problem by simply raising the glass transition temperature to a temperature exceeding 100° C. was not unsuccessful. Thus, the sufficient property for storage at high temperatures has not been achieved. Moreover, a problem arises in that exciplexes are formed by the interaction between the hole transporting layer and the light emitting layer and the luminance of the device deteriorates.